1. Field of the Invention
The invention relates generally to mechanisms and methods for closing off a subsurface wellbore and, in particular aspects, to surface-controlled subsurface safety valves used within wellbores.
2. Description of the Related Art
Surface-controlled subsurface safety valves (SSSVs) are used to selectively close off lower portions of the flowbore of a production tubing string in the event of an emergency. These valves can then be reopened later when the emergency situation has been remedied and it is desired to reestablish flow through the flowbore. Common SSSV's are flapper-type valves that are biased closed by a spring mechanism and then reopened, under surface control, by an axially moveable flow tube. Although flapper valves have been in use for a long period of time, they continue to suffer from problems that prevent them from being an ideal solution in all situations. Because they are mechanical devices, their components are prone to damage during typical operation. Also, the flow path might become encrusted with scales or hydrates during production, which can cause the valve to become stuck in an open, closed, or partially open position.
Certain devices are known that utilize plugs that will block fluid flow through the flowbore and can be readily destroyed when needed. U.S. Pat. No. 6,026,903 issued to Shy et al., for example, discloses a frangible disappearing plug that can be ruptured and destroyed to reopen the flowbore of the well. However, this type of structure must be run into the well at the outset in order to be useful as a plug. This makes it unsuitable for use as an SSSV, since it could not be used to rapidly close off a lower portion of the flowbore in an emergency.
There is a need for an alternative to standard flapper-type surface controlled safety valves. There is a need for an alternative technique that can be used to establish a blockage within the flowbore of a well during operation in a substantially rapid manner under surface control to close a portion of the flowbore in the event of an emergency. This technique should also allow the flowbore to be reopened, if desired.
The present invention addresses the problems of the prior art.